1. Field of the Invention
The invention relates to a printing device comprising a printing unit and paper feed rollers arranged in the paper supply direction before the printing unit as well as a paper detector arranged between the printing unit and the paper feed rollers and sensing there the presence of paper.
2. Description of the Prior Art
Such a device is known from DE OS 2146451 which corresponds to U.S. Pat. No. 3,734,011. For controlling the feeding of document cards several detectors are provided therein at different areas of the transport path, which indicate the presence of the document card at these areas or the passage of an edge.
Printing devices can be designed for printing separate sheets or endless paper. Separate sheets can be supplied manually or automatically from a supply stack by means of a separation device. Endless paper is mostly supplied from a folded stack by means of a tractor to the printing device. The paper must be supplied in the correct position to the paper feed rollers of the printing device. The steps of transferring and passing on the paper by the paper feed rollers must be timed with respect to the printing unit in order that the paper can be printed at the desired areas. Especially if several paper transport paths of the kind described are assigned to a printing device, in known solutions numerous paper detectors are required, which should control more particularly the following functions:
the advance feed path of the paper to the first printing line, PA1 the transport path of separate sheets to the stacker, PA1 the transport path of endless paper to the perforated tearing edge, PA1 the return path of endless paper to a parking position, the start of the automatic paper feeding with manual supply of separate sheets. PA1 the indication that endless paper has left, PA1 the monitoring of the paper transport, PA1 the recognition of the presence or absence of paper, PA1 the recognition of a jamming of paper, more particularly with endless paper, PA1 the stopping of printing when the back edge of the paper enters prematurely (for example with too short a form length), PA1 measurement of the paper length.